PEP: technical details¶

Telebit never published PEP. This page records what D-Modem's version (dsp/pep.h, dsp/pep.cpp, the pump dsp/dp_pep.cpp) is based on and what it had to make up. For an overview, see PEP.

Where it comes from¶

• US 4,679,227 (Hughes-Hartogs, 1987), "Ensemble modem structure for imperfect transmission media": the ensemble, the start-up's eight steps, the bit loading, the tracking. Most numbers below are its.
• US 4,731,816 and 4,833,706 (Hughes-Hartogs), its continuations, and US 4,438,511 (Baran), the packet modem behind Adaptive Duplex.
• The T2500's reference manual (1991): the S registers that report PEP's state (S70 to S77), S50, S92, the result codes.
• Two Telebit notes of 1990, Ballard's overview and one on PEP on poor lines: the rates, the packet sizes, Adaptive Duplex.

No firmware image of a TrailBlazer, T1000, T2000 or T2500 has been found, nor a recording of a call made with the line demodulated, so nothing below has been checked against a Telebit.

The ensemble¶

The two timing tones, 1437.5 Hz and 1687.5 Hz (bins 184 and 216), are the patent's. In data epochs D-Modem keeps their carriers as pilots at the comb's level (its own choice).

The start-up¶

The patent's steps, in order:

1. The answerer sends an epoch of the two timing tones at -3 dBR each, then a comb of all 511 carriers at -27 dBR (D-Modem: 4 comb epochs, their signs - + - +, so that the last is the one the receiver's timing locks to and the others measure how steady each carrier is).
2. Both measure the line's noise in silence (3 FFTs).
3. The caller sends its tones and comb the same way.
4. The caller sends a phase coded epoch at -28 dBR: 180 degrees on the carriers that take 2 bits the answerer's way, 0 on the others (D-Modem sends it twice, for the margin).
5. The answerer replies with the carriers that take 2 bits both ways (D-Modem: in the same form, at the start of a turn).
6. On those carriers (the "link"), each side sends the other, in packets, the bits and power of each carrier its way.

A carrier takes b bits when its SNR (the channel's power against the larger of the silence's noise and the comb's unsteadiness) is the QAM's need for b bits plus a margin of 3 dB. The loading follows Hughes-Hartogs: the next bit always goes where it costs the least power, until the comb's power is spent. The loading record (D-Modem's own) is 'L' and 10 bits a carrier: 4 for its bits, 6 for its power against the comb's, -8 dB up in 0.25 dB steps. The call is up (CONNECT) once each side has had a turn of the other's on its own loading.

Turns and packets (D-Modem's own)¶

A turn is 20 ms of silence, 32 ms of both timing tones, one comb epoch (its sign telling the loading: + the link, - the final one) and then one packet an epoch, at least 1 and at most 10 (turn_max, about 3 KB at the top rates). The receiver finds the tones' end in 128 sample blocks, then the comb's timing and channel; each data epoch's timing and phase come from the decided points by least squares, and the channel and the far side's clock are followed from them. The window is interpolated on the far side's clock.

A packet is

all scrambled. The window is 64 packets, with selective retransmission: a turn starts with the packets the far side has not acknowledged. A side with nothing to send sends one fill packet (a micro turn, below, once on the final loading), so the other side gets the line back at once (Adaptive Duplex). If no turn comes (the far side's lost), the caller takes the line again after 0.6 s, the answerer after 0.9 s, unless the line carries more than its quiet level (the far side in a turn whose tones were missed).

Micro turns (D-Modem's own)¶

Telebit's micro packets run at 88.26 baud on a few carriers; their layout is unknown. D-Modem's micro turn is 20 ms of silence, 24 ms of both tones (PRE_SHORT), then four short epochs of 64 samples and 26 of guard (90 samples, 88.9 baud): one short comb, then three data epochs of one packet. A short epoch uses every 16th carrier (125 Hz apart, 375 to 3375 Hz), 4 times the long epoch's amplitude (16 times the power, as the long epoch's power is spread over 16 times as many carriers), so each takes 2 bits more than the final loading gives that carrier, 6 at most. The short symbol is the first 64 samples of the 1024 point IFFT with only those bins, so the long comb's channel applies to it unchanged.

A side sends a micro turn when it has no packet to send again, no control data and less data than one micro packet holds (about 50 octets at 30 dB). A receiver on the final loading tries every turn as a micro turn first: it scans 160 samples from the tones' end for the short comb (the adjacent carriers' phase steps, against the channel, coherent above 0.8), refines its timing from their mean, takes the common phase from it and decodes the packet; a bad CRC (a long turn) goes on to the long comb. A micro turn takes about 90 ms against 330 ms for a turn of one long packet.

The pump and the link¶

The pump (dp_pep.cpp) gives PEP the link layer's bits as octets. When the link frames them (V.42, MNP or start-stop characters, which the core says with dp_runtime_set_framed()), the pump takes them at the rate the link was given at CONNECT (so its bit timers keep time), up to two long turns ahead, and leaves out its idle: ones after 9 ones kept (no character or frame has more) and flags after 16 kept (V.42's answerer needs a few to see one as a character, and asynchronous characters never repeat one 8 bit pattern three times). The received octets go to the link at the far side's line rate, not a turn's at once. A synchronous call's bits are a raw stream and go as they are.

The caller skips V.42's detection phase in PEP and starts LAPM at once: the ODPs and the answer to them would take two turns each way, past T400 (750 ms). The answerer keeps its detection phase, which ends on the caller's flags, or takes MNP's LR.

Automode and the answer tone¶

The caller's watch, before PEP's tones, takes the line in 10 ms blocks: the power at 2100 Hz, at the two tones, and the answer tone's envelope. Its AM depth over 400 ms (the envelope less its mean, against it; above 0.1) tells ANSam from a plain answer tone, which V.34's caller now measures the same way. The answer tone survives ANSam's phase reversals (gaps of up to 5 blocks). After it, 150 ms of a signal that is neither PEP's tones nor 2100 Hz is another modulation. The pump hands the call over with DPSTAT_CHANGEDP (V.34 for ANSam and for the answerer, V.32bis with its automode otherwise, Bell 212A for 2225 Hz), and the call's statistics are no longer PEP's.

Statistics¶

At CONNECT and on each block after it the pump fills the call's statistics: the line rates, the packets' counts, the micro turns, and for each of the 512 bins the bits each way and the noise measured in the start-up (noise_dbm0()). ATI11 shows the counts, ATI12 the table (Telebit's S70 to S75 would clash with D-Modem's own S70, S71 and S92, so they are not S-registers).

What the T2000 recordings show¶

Five recordings of a Telebit T2000 (Wikimedia Commons: call set-up, duplex and simplex data, XMODEM with and without spoofing) were measured:

• The answerer sends 3.1 s of plain 2100 Hz and 70 ms of silence before PEP's tones.
• The epoch is 1024 samples at 8000 Hz plus a guard of about 61 (64 in Telebit's documents; the lossy audio makes it rough), the guard a copy of the symbol's start after it. The comb (E0) is the same both ways, and so are the 9 training epochs.
• The T2000's carriers end at bin 434 (3390.6 Hz).
• A turn starts with one E0 epoch and no tones; on the 2-bit link the data is QPSK times E0's phases.
• Long turns are about 1.8 s. An idle line is a 46 ms burst each way every 120 ms, about a dozen carriers each; their epoch could not be found.

No turn was decoded to bits, so the packets, the scrambler and the loading record of a real Telebit are still unknown.

Not done¶

• S36's lead tone, S120; Telebit's small packets.
• Telebit's compression (S110), UUCP g, Kermit and XMODEM spoofing (S111).
• Turbo PEP (the WorldBlazer's, 7 bits a carrier, 23 kbit/s).
• Talking to a real Telebit (its packets are not known).